1. Field of the Invention
This invention relates to a new class of transition metal containing linear polymers of varying molecular weight that are useful for conversion to high temperature thermosets and ceramics. These new materials have repeat units that contain alkynyl groups for cross-linking purposes along with organotransition metal complexes, silyl, siloxyl, boranyl, or di(silyl or siloxyl)carborane units. These novel linear polymers with the metal units in the backbone are soluble in most organic solvents and can be easily fabricated from the melt into shaped components, which enhance their importance for high temperature structural, magnetic, and microelectronic applications. Cross-linking of alkynyl groups is known to occur by either photochemical or thermal processes. The ceramics may also contain carbon nanotubes and/or metal nanoparticles.
2. Description of the Prior Art
The incorporation of transition metals into a polymer structure has long been seen as a good way of preparing materials with different properties from conventional carbon-based polymers. Small molecule transition metal complexes and solid state compounds possess an array of interesting high temperature, hardness, redox, magnetic, optical, electrical, and catalytic properties. In addition, the rich diversity of coordination numbers and geometries available for transition elements offer the possibility of accessing polymers with unusual conformational, mechanical, and morphological characteristics.
The development of polymers with transition metals in the main chain structure would be expected to provide access to processable, specialty materials with similarly attractive physical properties that would be of interest as pyrolytic precursors to metal-containing ceramics. Transition metal-based polymers might also function as processable precursors for making metal-containing ceramic films and fibers with high stability and desirable physical properties. Most transition metal-based polymers reported to date, however, do not contain units for conversion to a thermoset and thus afford low char yields at elevated temperatures.
Despite early synthetic problems of constructing macromolecular chains, researchers have now prepared a variety of metal-containing polymers with novel properties. Ferrocene-based polymers appear to be particularly promising as reported by Ian Manners in Chain Metals, Chemistry In Britain, January 1996, pp. 46-49. Because of ferrocene""s ability to release and accept an electron reversibly, there is considerable interest in developing these materials as electrode mediators and in energy storage devices.
These mediators, for example, facilitate electron transfer between an enzyme such as glucose oxidase, where the redox active sites are buried in a protein sheath and an electrode. Ferrocene-based polymers have been successfully used as electron relays in electrochemical biosensors for measuring glucose levels. Scientists have also fabricated microelectrochemical devices such as diodes using ferrocene-based polymers.
Other studies have reported on the formation of Fexe2x80x94Sixe2x80x94C materials from the pyrolysis of iron containing polymers. See, for example: (1) Tang, B. Z.; Petersen, R.; Foucher, D. A.; Lough, A.; Coombs, N.; Sodhi, R.; Manners, I. J Chem. Soc., Chem Commun. 1993, 523-525; (2) Peterson, R; Foucher, D. A.; Tang, B. Z.; Lough, A.; Raju, N. P.; Greedan, J. E.; Manners, I. Chem. Mater. 1995, 7, 2045-2053; and (3) Ungurenasu, C. Macromolecules 1996, 29, 7297-7298; (4) Hodson, A. G. W; Smith, R. A. Transition Metal Functionalized Polysilazanes as Precursors to Magnetic Ceramics, Faculty of Applied Sciences, University of the West of England, Bristol, BS16 1QY.
Spirocyclic [1]-ferrocenophanes have been reported to function as convenient cross-linking agents for poly(ferrocenes) via thermal ring-opening copolymerization reactions by MacLachlan, M. J.; Lough, A. J.; and Manners, I, in Macromolecules, 1996, 29, 8562-8564.
The use of ring-opening polymerization (ROP), a chain growth process, is reported by I. Manners in Polyhedron, Vol. 15, No. 24. pp 4311-4329, 1996 to allow access to a range of high molecular-weight polymers with skeletal transition metal atoms having novel properties.
Several poly(ferrocenylsilanes) have been synthesized and converted into ceramics upon heating to 1000xc2x0 C. under inert conditions. See, for example, Pudelski, J. K.; Rulkens, R.; Foucher, D. A.; Lough A. J.; MacDonald, P. M. and Manners, I., Macromolecules, 1995, 28, 7301-7308. The ceramic yields by thermogravimetric analysis (TGA), however, were in the range of 17 to 63%.
Alternative transition metals such as ruthenium have also been reported as being incorporated into a metallocenophane structure by Nelson, J. A.; Lough, A. J., and Manners, I in xe2x80x9cSynthesis and Ring-Opening Polymerization of Highly Strained, Ring-Titled[2]Ruthenocenophanesxe2x80x9d Angew. Chem., Int. Ed. Engl. 1994, 33, 989-991 and in xe2x80x9cSynthesis, Structures, and Polymerization Behavior of Di-silane-Bridged and Bis(disilane)-Bridged[2]Ruthenocenophanesxe2x80x9d in Organometallics 1994, 13, 3703-3710. Novel ruthenium or iron containing tetraynes as precursors of mixed-metal oligomers are reported in Organometallics 1996, 15, 1530-1531. Mixed valence diferrocenylacetylene cation compounds have been reported in the Journal of the American Chemical Society, 96:21, 1974, pp. 6788-6789.
The synthesis and characterization of linear boron-silicon-diacetylene copolymers is reported by R. A. Sundar and T. M. Keller in Macromolecules 1996, 29, 3647-3650. Additionally, the efficient, xe2x80x9cone-potxe2x80x9d synthesis of silylene-acetylene and disilylene-acetylene preceramic polymers from trichloroethylene is reported in the Journal of Polymer Science: Part A: Polymer Chemistry, Vol. 28, 955-965 (1990).
Furthermore, the preparation and reactions of decachloroferrocene and decachlororuthenocene is disclosed in the Journal of the American Chemical Society, 95, 870-875 (1973). Symmetrically disubstituted ferrocenes are discussed in the Journal of Organometallic Chemistry, 27 (1971) pp. 241-249, as well as ferrocenyl-acetylene being disclosed in the J. Organometal. Chem., 6 (1966) pp. 173-180 and 399-411. Ferrocenyl- and 2-thienylarylacetylenes are reported by M. D. Rausch; A. Siegal and L. P. Kelmann in J. of Org. Chem. 1966, Vol. 31 p. 2703-2704.
Ferrocenyl ethylene and acetylene derivatives are also reported by P. L. Pauson and W. E. Watts in J. Chem. Soc. 1963, 2990-2996. Studies on the reactions of ferrocenylphenylacetylene and diferrocenyl-acetylene are reported in the Journal of Organometallic Chemistry, 149 (1978) 245-264. The chemistry of xcfx80-bridged analogues of biferrocene and biferrocenylene is discussed in the Journal of Organic Chemistry, Vol. 41, No. 16, 1976, 2700-2704. The synthesis of 1xe2x80x2,6xe2x80x2-bis(ethynyl)-biferrocene and metal complexes referring to non-linear optics is presented in Polyhedron, Vol. 14, No. 19, pp. 2759-2766 (1995).
In addition to these documents discussing the various compounds and polymers, the catalytic graphitization by iron of isotropic carbon is reported in Carbon, Vol. 21, No. 1, pp. 81-87, 1983. Preceramic polymer routes to silicon carbide are disclosed by Richard M. Laine in Chem. Mater. 1993, 5, 260-279; and the comprehensive chemistry of polycarbosilanes, polysilazanes and polycarbosilazanes as precursors of ceramics is thoroughly reported in Chem. Rev., 1995, 95, 1443-1477.
U.S. Pat. Nos. 4,800,221 and 4,806,612 also respectively disclose silicon carbide preceramic polymers and preceramic acetylenic polysilanes, which may be converted into ceramic materials.
U.S. Pat. Nos. 5,241,029 and 5,457,074 disclose diorganosilacetylene and diorganosilvinylene polymers that can be thermally converted into silicon carbide ceramic materials.
U.S. Pat. No. 4,851,491 discloses polyorganoborosilane ceramic polymers, which are useful to generate high temperature ceramic materials upon thermal degradation. U.S. Pat. No. 4,946,919 also relates to boron-containing ceramics formed from organoboron preceramic polymers, which are carboralated acetylenic polymers.
U.S. Pat. Nos. 5,272,237; 5,292,779; 5,348,917; 5,483,017 disclose carborane-(siloxane or silane)-unsaturated hydrocarbon based polymers reported to be useful for making high temperature oxidatively stable thermosets and/or ceramics.
U.S. Pat. No. 5,552,505 discloses copolymers formed from aromatic acetylenic monomers or prepolymers formed therefrom and carborane-(siloxane or silane)-unsaturated hydrocarbon polymers reportedly useful to form articles, adhesives, matrix materials, or coatings, or which may be pyrolyzed to form carbon-ceramic composites. Each of the documents cited herein contains valuable information and each is incorporated herein by reference in its entirety and for all purposes.
Most of the carborane-siloxane and/or carborane-silane polymers made by others have elastomeric properties rather than properties of more rigid polymeric products like thermosetting polymers or ceramics. There is a need for polymers that behave less like elastomeric polymers and more like thermosets and which, upon pyrolysis, form ceramics.
U.S. patent application Ser. Nos. 10/216,469 to Keller et al. and 10/216,470 to Keller et al., incorporated herein by reference, disclose the formation of carbon nanotubes and metal nanoparticles from the pyrolysis of compositions containing metallocene, ethynyl groups, and/or metal ethynyl complexes. These complexes did not contain any silicon. Upon heating, the ethynyl groups crosslinked to form a thermoset. Further heating caused the metal-containing groups to degrade releasing elemental metal. This process can catalyze the formation of carbon nanotubes in the bulk material. The metal can also coalesce into nanoparticles.
There is, therefore, a need for oxidatively stable materials having thermosetting properties for making rigid components therefrom which withstand high temperatures and which have high strength and high hardness properties and/or which optionally may have magnetic properties.
Furthermore, there is a need for transition metal-based polymers that contain units for conversion to thermosets and which afford high char yields at elevated temperatures. There is also a need for such polymers which would be useful precursors to novel materials unavailable from other sources, and which may exhibit unique nonlinear optical (NLO) properties.
There is a further need for materials containing silicon and carbon nanotubes or metal nanoparticles in the bulk material.
It is therefore an object of the present invention to provide polymers having backbones incorporated with organotransition metals complexes along with silicon, acetylenic, and/or boron units which are useful as precursors to novel materials unavailable from other sources.
It is another object of the present invention to provide polymers having backbones incorporated with organotransition metal complexes along with silicon, acetylenic, and/or boron units which can be readily converted into high temperature thermosets.
It is another object of the present invention to provide polymers having backbones incorporating organotransition metal complexes along with silicon, acetylenic, and/or boron units which can readily be converted into high temperature materials which exhibit high strength properties, high hardness values, and electrical and/or magnetic properties.
It is yet another object of the present invention to provide transition metal based polymers that contain inorganic units and units for conversion to thermoset polymers and which afford high char yields at elevated temperatures.
It is still another object of the present invention to permit the formulation of ceramics containing a variable and controllable amount of metal and various cluster sizes.
It is still another object of the present invention to provide transition metal based ceramics that contain inorganic units and carbon nanotubes and/or metal nanoparticles.
These and other objects of the invention may be accomplished by a transition metal-containing ceramic made by the process comprising the step of pyrolyzing an organometallic linear polymer containing at least one metallocenylene unit, at least one silyl or siloxyl unit, and at least one acetylene unit to form a ceramic; where the ceramic has a ceramic yield of at least about 75% by weight.
The invention further comprises a transition metal-containing ceramic made by the process comprising the steps of: forming an organometallic linear polymer containing at least one metallocenylene, at least one silyl or siloxyl unit, and at least one acetylene unit; crosslinking said linear polymer through the acetylene units, thereby forming a thermoset; and pyrolyzing said thermoset to form a ceramic; where the ceramic has a ceramic yield of at least about 75% by weight.